User input devices such as mice and trackballs for use with computers and other electronic devices commonly use opto-mechanical encoding to sense position and/or movement. Motion of the ball of a mouse or trackball, or motion of the wheel and/or shaft of a thumb- or finger-wheel, for example, typically rotates a pair of encoding wheels having light-transmitting and light-blocking regions
Each encoding wheel is typically positioned between one or two light sources in the form of LEDs (light emitting diodes) and two light sensors in the form of PTRs (phototransistors). For each PTR, the surface area of the PTR exposed to the light from the LED(s) is directly correlated with the position of the encoding wheel, and may be approximately represented by a periodic function of the position of the encoding wheel. The signal voltage at the emitter of each PTR is, in typical configurations, directly proportional to the surface area of the PTR exposed to light. The light source(s) and the two PTRs are typically positioned, relative to each other and to the encoding wheel, such that the signal voltage from one PTR varies approximately in quadrature with the signal voltage from the other PTR, as a function of the position of the encoding wheel. The signals from the two PTRs, taken together, are thus representative of the velocity and direction of motion of the encoding wheel: the frequency of the signals indicates the velocity, and the relative phase indicates the direction, of the encoding wheel.
A typical input device includes at least two (one for each of two orthogonal directions of ball rotation), and often three encoding wheels, resulting in the use of four or six PTRs, and a minimum of 2 or 3 LEDs. A microcontroller is employed to control and interpret the sampling of the PTRs and to provide communication with a host device.
Some optical encoding methods require sample times of 50-100 .mu.s or more. Many traditional opto-mechanical encoding systems, while reliable and low-cost, may require as much as a 3-4 mA average, and 10 mA peak, current. Minimizing this current draw provides an important advantage in battery-operated or parasitically powered devices. Long sampling intervals tend to result in higher average current than short sampling intervals. Long sampling intervals also limit the tracking performance. Shorter intervals allow higher-resolution tracking, or higher-speed tracking without aliasing, or both.